The production of alumina by the Bayer process involves three main steps: digestion of bauxite, precipitation of aluminium trihydrate (gibbsite, Al(2)O(3).3H(2)O) from the resulting solution and calcining of the gibbsite at 980-1200 degrees Celsius to produce alumina. The calcination step absorbs 3GJ/t alumina or 25% of the total Bayer process. In the current project a highly innovative modification of the Bayer process has been developed, whereby aluminium monohydrate (boehmite, Al(2)O(3).H(2)O) rather than the trihydrate is the product of the precipitation stage.
In this process boehmite is precipitated under conditions similar to these used for gibbsite precipitation (atmospheric conditions at temperatures lower than 100 degrees Celsius). This results in significant energy savings, estimated to 1.8 GJ/t alumina or 60% over the current practice at the calcination stage because (a) the enthalpy of dehydration of boehmite to alumina is lower by 1.1 GJ/t alumina compared to that of gibbsite, and (b) the quantity of material to be calcined will be reduced by 20% per t alumina, as only one rather than three moles of water are associated to one mole of alumina. Because of this huge energy savings potential, it is expected that the proposed Bayer process modification will be applied to all new smelters; it is also possible that the incentive will be high enough so that it will be adopted by existing smelters as well, with the appropriate modifications in the precipitation section.
The exploitable result of this project is a new process in which boehmite is precipitated, isothermally or not isothermally, from sodium aluminate solutions at atmospheric conditions and at temperatures lower than 100 degrees Celsius. The project is addressing the development of an energy efficient process (B07) and generally to Process engineering (E33).
This process is a highly innovative variation of the Bayer process and has not been applied before in the alumina industry or in any other relevant industries. It comprises a modification of the precipitation section of the Bayer process whereby aluminum monohydrate (boehmite, Al(2)O(3).H(2)O) rather than trihydrate (gibbsite, Al(2)O(3).3H(2)O) is precipitated. This will result in energy savings of 1.8 GJ/t alumina; for an average size smelter producing 1.000.000 t alumina/year, this represents savings of 1.8x10(-6) GJ/year.